Safety apparatus for nuclear reactor to prevent structural damage from overheating by core debris

ABSTRACT

The invention teaches safety apparatus that can be included in a nuclear reactor, either when newly fabricated or as a retrofit add-on, that will minimize proliferation of structural damage to the reactor in the event the reactor is experiencing an overheating malfunction whereby radioactive nuclear debris might break away from and be discharged from the reactor core. The invention provides a porous bed or sublayer on the lower surface of the reactor containment vessel so that the debris falls on and piles up on the bed. Vapor release elements upstand from the bed in some laterally spaced array. Thus should the high heat flux of the debris interior vaporize the coolant at that location, the vaporized coolant can be vented downwardly to and laterally through the bed to the vapor release elements and in turn via the release elements upwardly through the debris. This minimizes the pressure buildup in the debris and allows for continuing infiltration of the liquid coolant into the debris interior.

CONTRACTURAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant toContract No. No. W-31-109-ENG-38 between the U.S. Department of Energyand the University of Chicago representing Argonne National Laboratory.

This is a continuation of application Ser. No. 472,368 filed Mar. 4,1983, now abandoned.

BACKGROUND OF THE INVENTION

A nuclear reactor generally has a vessel within which a reactor core ispositioned and primary coolant flows. Commercial reactors commonly usewater as the primary coolant, although reactors are operative whereliquid metal sodium is used as the primary coolant. The reactor coreitself has a large network of elongated passageways and nuclear fuel inthe form of elongated fuel pins is located within certain of therespective passageways according to a specific cross sectional array.Neutron absorbing elements are likewise formed as elongated members andare positioned in certain of the other passageways, again according to aspecific cross sectional array. The precisely defined matrix between thevarious fuel pins and neutrons absorbers and the controlled extent oflateral overlap or relative degree of presence of each within the matrixdetermine the extent and rate of the nuclear fission in the reactorcore. Control rods extending into the reactor vessel are used to movethe fuel pins and/or the neutron absorbers axially relative to oneanother to achieve the intended control overlap.

The primary coolant is forced through the reactor core passageways anddirected then by appropriate piping or line means to heat exchangers,wherein a different or secondary coolant removes the heat from theprimary coolant. The primary coolant in turn is circulated back to thereactor vessel for passage again through the reactor core passageways.The secondary coolant, generally pressurized water, is vaporized andthis high pressure steam is expanded through steam driven turbines whichpower generator equipment that produces useful electrical energy.

The fuel pins might commonly be formed of a uranium oxide or plutoniumoxide, cladded with a durable material such as zirconium or stainlesssteel; and the reactor core itself might be fabricated of stainlesssteel. The reactor vessel immediately below the reactor core typicallyhas a gridwork therein for directing the primary coolant upwardly froman underlying plenum space into and through the passageways of thereactor core. Pumps of course normally would force the coolant into theplenum.

In the event the reactor should malfunction and begin to overheat, thepossibility exists that the fuel pins or the cladding on the fuel pinsmight fracture and/or melt and interact with the coolant to form smallparticles of nuclear debris which would drop or fall then through thecoolant to the underlying gridwork or reactor vessel. The nuclear debriscould reach significant heights, as measured in terms of centimeters,and represent the problem within which this invention is directed.

The cause of overheating or malfunction could be reduced circulation ofthe coolant, even though the entire plenum may yet be, and veryfrequently will be, filled with the liquid coolant. The nuclear debriswill be highly radioactive and thus will generate heat having a highheat flux. The coolant directly overlying the debris will cool at leastthe exterior particles of the debris, and convective circulation withinthe plenum space will normally be sufficient to keep the coolant at thedebris surface in the liquid phase. Inasmuch as the coolant cannotrapidly get to all of the interior particles of the debris because ofthe torturous or long flow paths the coolant must take to reach theseparticles, the coolant frequently can be vaporized only slightly belowthe debris surface. The vaporized coolant normally moves upwardlythrough the overlying particles in the pile, and thus further hindersliquid coolant penetration into the debris interior for cooling theparticles thereat. This depletion of coolant toward the interior of thedebris allows localized "pile dryout", where temperatures up to possiblyeven 3000° C. can be generated. These dryout temperatures generallyexceed the design limits of the structural materials forming the reactorvessel and cavity, which thus could be structurally damaged. Destructionof the reactor vessel and the containment vessel, could lead to theescape of the coolant and the nuclear debris from such confinement,which in turn could allow the release of detrimental radioactivity tothe environment and its resultant threat to public safety. Repairs ofthe reactor would, of course, also be most costly.

SUMMARY OF THE INVENTION

This invention relates to a safety feature to be incorporated in anuclear reactor so as to minimize the extent of damage that mightotherwise occur in the event of a reactor overheating malfunction, suchas the structural failure of reactor components and the subsequentpossible detrimental and unsafe release to the atmosphere of radioactiveemission or the like.

A basic object of this invention is to provide apparatus which can beincorporated directly into the reactor, either during its initialconstruction or as a retrofit add-on to an existing reactor cavity, thatwould provide effective cooling of nuclear debris generated because ofthe partial destruction of the nuclear fuel pins and/or the reactor coreitself as a result of an overheating malfunction of the reactor. In suchan accident scenario, the debris would normally pile up in the reactorand generate heat sufficient to melt through or destroy the reactorvessel and/or reactor containment structure and allow thereby the escapeof the coolant and debris from such containment whereby radioactiveemission could be discharged also to the ambient atmosphere.

A specific object of this invention is to provide a porous bed in thereactor vessel and/or in the containment vessel or cavity, generallyunderlying the reactor core, such that any nuclear debris generated inand discharged from the core will fall upon and pile up on the bed. Aplurality of elongated porous vapor release elements upstand from thebed at laterally spaced locations, terminating at their lower endswithin the beds and at their upper ends sufficiently above the bed toremain exposed even when debris may fall onto the bed. The porous bedallows lateral transfer of coolant vapor through it from one location toanother, and the porous vapor release elements allow vapor to passtherethrough axially along their length from within the bed to above thedebris. In this manner, liquid coolant overlying the nuclear debris canreadily circulate by gravity into the interior of the debris, and anycoolant vapor generated because of the high heat flux of the debris canescape downwardly to the bed and then laterally within the bed andthrough to the adjacent upstanding vapor release elements, so as topreclude localized pile dryouts in the accumulated debris.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross sectional view of a reactor, illustratingthe core and its containment structure, and showing a preferredembodiment of the subject invention incorporated therein;

FIG. 2 is an enlarged sectional view, as seen generally from line 2--2in FIG. 1, illustrating one embodiment of the vapor release elements,where for clarity of disclosure, two of the three illustrated elementsare in slightly different sectional orientations and the third elementis in elevation;

FIG. 3 is the top plan view of the vapor release elements illustrated inFIG. 2;

FIGS. 4, 5, 6, 7 and 8 are elevational sectional views, similar to FIG.2, except showing different embodiments of the particular invention;

FIG. 9 is a sectional view as seen from line 9--9 in FIG. 8; and

FIG. 10 is an elevational cross sectional view, similar to FIG. 1,except showing a modified reactor design and a different embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 10 each schematically illustrates a nuclear reactor 10(10a), the reactor having having a core 12 (12a) disposed within aclosed vessel 14 (14a)--only the bottom portion of each being shown, thecore 12 (12a) having vertically extended passageways 16 (16a) withinwhich fuel elements and control elements (neither being shown) arepositioned. A plenum space 18 (18a) is defined within the vessel 14(14a) underlying the reactor core 12 (12a), and inlet pipes 20 (20a)allow for conveying the primary coolant to the plenum space. Pump means(not shown) circulate the primary coolant upwardly from the plenum space18 (18a) through the core passageways 16 (16a) to a heat exchanger (notshown) wherein the heat of the nuclear reaction in the core 12 (12a)isgiven off to the secondary coolant. As noted above, the secondarycoolant is generally in the form of steam that is expanded throughturbine-like expansion means to generate electrical power. Gridworkstructure 21 (21a) located in the plenum 18 (18a) directs the coolantupwardly and into the defined core passageways 16 (16a); while supportstructure 22 (22a) suspends the core 12 (12a) from and within thereactor vessel 14 (14a).

The reactor 10 illustrated in FIG. 1 is typical of a water-cooledreactor (LWR), where the reactor vessel 14 is located within areinforced containment or cavity 28, having walls 30 formed of concreteand a steel liner 32 surrounding the lower part of the reactor vessel14. The primary coolant could be in the form of water confined under ahigh pressure (2000 psi). The reactor 10a illustrated in FIG. 10 mightbe a liquid metal cooled reactor (LMFBR), where the reactor vessel 14ais generally confined in a larger guard vessel 34a which in turn islocated then in the reactor containment or cavity structure 28a havingthe concrete walls 30a and steel liner 32a. The primary coolant would bein the form of sodium confined at approximately one atmosphere ofpressure which at the intended operating temperatures of between 250°and 550° C. is in the liquid phase. The space 36a between the reactorand guard vessels (14a and 34a) and/or space 37 (37a) between thereactor vessel and containment structure 14 and 28 (34a and 28a) can bemaintained under an inert gas atmosphere that can be monitored to detectany change in this atmosphere so as to detect or confirm if amalfunctioning leak has occurred.

The reactor vessel 14 (14a) id designed of sufficient strength towithstand both normal and malfunctioning pressures and/or temperaturesof the operating reactor, but the wall 38 (38a), particularly onexisting reactors, underlying the core 12 (12a) may not be of suchdurability to withstand the emergency malfunctioning scenario to whichthis invention is directed. Even the containment structure 28 (28a),which has been fabricated, even on existing reactors, as one of the lastcontainments to keep the reactor fuel and coolant components enclosed inorder to preclude the release of radioactive or otherwise harmfulemission, could always be attacked at the high temperatures of 3000° C.to release gases such as carbon dioxide and steam and further could alsobe disintegrated under this attack into its formative components.

Reactor overheating might be caused by excess generation of heat becauseof runaway fission, or by a reduction in the effectiveness of cooling,because of defective coolant pump means (not shown) or blockage of thecoolant flow paths, as possibly by a failed fuel pin swelling within thecore passageway 16. In the destructive overheating scenario, thegenerated nuclear debris from the the fuel pins and/or core 12 (12a)drops through the coolant and collects as a debris pile or layer on anunderlying horizontally disposed surface near or at the bottom of thereactor vessel. In FIG. 1, one layer is illustrated at 42 within thevessel 14 and a second layer 44 is illustrated in the containment cavity28; while in FIG. 10, only layer 46a is illustrated in the vessel 14a.Localized melting or destruction of the reactor vessel wall 38 caused bythe heat generating nuclear debris would result in failure of thereactor vessel (as shown by the opening 48 in the wall) and subsequentdumping of the debris and coolant onto the containment cavity floor. InFIG. 1 therefore, a serious accident has already occurred to the extentthat the reactor vessel 14 has failed and the coolant and nuclear debrishas escaped to the containment cavity 28; while on the other hand, theillustration of FIG. 10 shows the nuclear debris yet confined within thereactor vessel 14a.

The nuclear debris could go through several cycles of melting andquenching by the liquid coolant resulting in the debris being in theform of particles ranging anywhere from 0.01 to 10 mm in size. Thephenomena of dryout can occur even when the pile of debris is completelycovered by the coolant, if the heat generating capacity at some interiorlocation of the pile is greater than the heat needed to vaporize thecoolant that can filter through the outer portions of the pile and reachthe pile interior. Thus, if the heat flux inside the pile is sufficientto vaporize the local coolant, vapor then rising through and from thepile adds to the resistance of the coolant filtering into the interiorof the pile; whereby the heat buildup is self generating and expanding.Under such a runaway scenario, the pile temperature at some interiorlocation could exceed 2500°-3000° C., well above the temperature thestructural material of the reactor can withstand before being destroyedor melted.

The invention teaches the use of a plurality of separate elongated vaporrelease elements 50 that are arranged as an array across the bottom ofthe reactor cavity underlying the reactor vessel (FIG. 1) or across thebottom of the reactor vessel underlying the reactor core (FIG. 10). Eachelement would have a height sufficient to exceed the height of any pileor layer of debris that may accumulate in the area. The elements arefabricated in a manner to be porous to allow vapor to travel axiallyalong the length of the element from below the debris to the open spaceabove the debris. This allows the vapors to escape upwardly from theinterior of the debris pile or layer, which thereupon allows the liquidcoolant to enter the debris rapidly from above and penetrate through thedebris particles to wet or cool the interior portions of the debris.This maintains the debris pile within the relatively cool temperaturelimits of the coolant in the liquid phase.

As illustrated, the elements are secured together by tie rods 2, one setbeing located adjacent the upper end of each element and possibly also asecond set (not shown) being located adjacent the lower end of eachelement. As thus supported, the vapor release elements are verticallyoriented and horizontally spaced from one another according to someselected matrix arrangement, such as illustrated in FIG. 3 in thesquared array, and can be on perhaps center-to-center spacing of between75 and 300 mm. The vapor release elements are designed to be held inthis arrangement both during normal operation of the reactor and in theevent of reactor malfunction where core debris particles may fall ontothe vapor release elements.

In the preferred embodiments, a porous bed 56 (56a) is initially laid onthe floor of the containment cavity 28 (FIG. 1) or on bottom wall 30 ofthe reactor vessel 14a (FIG. 10). The bed preferably is formed of aplurality of solid or imperforate ball-like elements that are randomlystacked on one another while leaving a void fraction ranging from 0.25to 0.75. The elements can be spherical balls of either steel or ceramicmaterial, for example, ranging in size from 1 to 20 mm; and the overalldepth of the bed could be in the range of 50 to 500 mm.

The vapor release elements, in one series of embodiments, would be tubesof solid side wall construction inserted to a depth of 10 and 50 mm inthe porous bed. The tube wall can also be made porous by slotting it orby drilling holes through it at relatively close spacings. The lower endof the vapor release element would be open to allow vapors to breatheinto the element from under the bed.

In an alternate embodiment (FIGS. 8 and 9), each vapor release element501 might be formed of porous material such as a sintered metal or aceramic baked or glazed to be self-supporting and structural. The entireelement could have a generally homogeneous cross section from end toend, and have a porosity equivalent to the inert packed particles with0.25 to 0.75 void fraction.

It is noted that the upper end of the hollow solid wall tubular vaporrelease element 50 is arranged to preclude the possibility of any debrisparticles from entering into the hollow interior of the tube via theopen top end. In one embodiment illustrated in FIGS. 2 and 3, aninverted cone 60 is supported by structural arms 62 as a roof spacedfrom but overlying the top of the tube structure 502. Alternatively, theupper end 64 (FIG. 4) of the tube may be oriented laterally or possiblyeven downwardly by having an elbow 66 secured to the tube, so as to makeit difficult or unlikely that falling debris will enter into the opentube end. Still further, the upper open end 68 (FIG. 5) of the tube 505may be closed by end cap 70; the tube wall then having small openings 72defined therein to give porosity to the tube end.

A further alternative embodiment provides that nonstructural looseparticles 76 (FIG. 6), such as the same loose ball or particle structurethat forms the bed 56, be located within the hollow of the structuraltube 506, stacked randomly on one another to leave multiple flow pathstherein for the passage of the vapor between the particles. This wouldyet allow the vapor to escape up the hollow interior of the structuraltube 506. Also, holes or slots 78 (FIG. 7) can be formed in the wallopenings of the tube 507, of size smaller than the particle 76 topreclude them from falling out through the openings; thus providing aporosity to the side wall of the tube 507 itself. The particle 76 in theopen top tubes 506, 507 would effectively preclude any of the debrisfrom entering into the interior of the tube.

The bottom ends of the upstanding porous elements can rest directly onthe horizontally disposed debris collection surfaces (where the nucleardebris would more than likely tend to accumulate) or be supported spacedoff of the surface by the particle bed disposed within the reactorvessel.

As noted, the tube construction need not have porous side walls but thetube can have impervious side walls and the tube interior communicatewith the open end of the tube disposed below the bed. Thus, by locatingthe end of the tube within the bed, vapors in the bed can breathethrough the tube interior and escape from the bed through the overlyinglayer or pile of debris. Under such circumstances, when debris collectson top of the bed, the porous bed laterally conveys any generatedcoolant vapors to the vapor release elements and the elements in turnvent the vapors through the debris.

With this bed configuration, the degree of porosity can be very closelyregulated by the depth of the bed and further by the size and shape ofthe balls or elements forming the bed. Spherical balls are idealvehicles for forming the bed as they can be randomly stacked or dumpedon one another to the desired depth while yet having many well-definedflow paths between the adjacent faces of the balls throughout the depthof the bed. While the bed and vapor release elements are illustrated inone embodiment in the containment vessel and in the other embodiment inthe reactor vessel, they could be provided in both vessels if desired.Thus, unless specifically identified differently, the term reactorconfinement in the claims is intended to cover either vessel.

By way of example, preliminary tests of a simulated pile of nucleardebris indicate vastly improved cooling rates that are 1.2-24 timesgreater than where no vapor release elements were used. Table 1summarized experiments in which a mixture of stainless steel and nickelparticles, of screened sizes between 0.1 and 1.0 mm, was used tosimulate the heat generating nuclear debris, the particles beinginternally heated by a 15-kW high frequency induction furnace. Glassballs were used to simulate the inert bed or sublayer, and variousliquid coolants were used. Heat flux of dryout rates, Q, were determinedby the condensation rate of the vapor in a condenser, and temperatureswere determined by thermocouples in the debris particles. Dryout heatfluxes higher than in the order of 950 kW/m² were beyond the capacity ofthe experimental apparatus and could not be measured accurately.Comparison of the dryout heat fluxes with the components of theinvention against those without demonstrate the substantial improvementthat the invention makes in maintaining the debris pile or layer cooland in otherwise avoiding the generation of localized pile dryouts.

These experiments also confirm that the vapor release elements used inconjunction with the porous bed below the heat generating debris allowcoolant vapor generated in the debris to move downwardly into the bedand then laterally through the bed to the vapor release elements andthen upwardly through the vapor release elements. This is an unexpectedphenomenon, and results in higher allowable dryout fluxes and thereforeprovides for greater margins for containment of heat generating debris.

These improved cooling rates are substantial and quite significant,particularly when comparing the cost of implementing the inventionagainst the possible dangers of failure and the costs of repair, whenthe improved cooling might not be available and the reactormalfunctions.

                                      TABLE 1                                     __________________________________________________________________________    EFFECT OF TUBES ON DEBRIS BED DRYOUT                                          Screened 0.1 mm to 1.0 mm Stainless Steel and Nickel Particles                Glass Ball Bed of Depth: 50 to 75 mm                                          Solid Wall Tubular Vapor Release Element: Glass                               Porous Vapor Release Element: 9.5 mm OD Stainless Steel 170 mesh                                        Average                                              ElementReleaseVapor                                                                 Dia., mmBallGlass                                                                  mmDepthDebris                                                                     Coolant    kW/m.sup.2Q.sub.dryoutMeasured                                                     kW/mm.sup.2Q.sub.dryoutComparative                                                   ##STR1##                               __________________________________________________________________________    None  No Bed                                                                             150 water      148  148    --                                      Porous                                                                              No Bed                                                                             150 water      >914 148    6.2                                     Mesh                                                                          Porous                                                                              No Bed                                                                             200 water      967  --     --                                      Mesh                                                                          None  5    150 water      102  102    --                                      Solid Wall                                                                          5    150 water      277  102    2.7                                     3 mm I.D.                                                                     None  3    100 water      265  265    --                                      Solid Wall                                                                          3    100 water      913  265    3.4                                     9 mm I.D.                                                                     None  3    100 water-ethylene glycol                                                                    223  223    --                                      Solid Wall                                                                          3    100 water-ethylene glycol                                                                    >956 223    4.3                                     5 mm I.D.                                                                     None  3    100 water-glycerine                                                                           40   40    --                                      Solid Wall                                                                          3    100 water-glycerine                                                                          >960  40    >24                                     5 mm I.D.                                                                     None  3    100 isopropyl alcohol                                                                         70   70    --                                      Solid Wall                                                                          3    100 isopropyl alcohol                                                                        143   70    2.0                                     9 mm I.D.                                                                     Porous                                                                              No Bed                                                                             100 isopropyl alcohol                                                                        144   70    2.1                                     Mesh                                                                          None  3    150 isopropyl alcohol                                                                         68   68    --                                      Solid Wall                                                                          3    150 isopropyl alcohol                                                                         83   68    1.2                                     9 mm I.D.                                                                     Porous                                                                              No Bed                                                                             150 isopropyl alcohol                                                                        104   68    1.5                                     Mesh                                                                          Porous                                                                              No Bed                                                                             200 isopropyl alcohol                                                                        116  116    --                                      Mesh                                                                          __________________________________________________________________________

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A nuclear reactorcomprising a core and a confinement vessel surrounding the core, fuelmeans in the core, liquid coolant for circulating over the fuel meansand through the core, and a apparatus within the confinement vessel forminimizing structural damage to the reactor in the event of a reactoroverheating malfunctioning accompanied by discharge of debris from thecore, said apparatus comprising a bed of particles with diametersessentially in the range of 1-20 mm, on a lower surface in thecontainment vessel and located beneath the reactor core, the bed beingporous to allow vapor to be passed laterally through it, and a pluralityof elongated, tubular elements, each having an interior passageway,disposed with the lower ends thereof terminating within the porous bedand the upper ends, thereof terminating in the reactor confinement at anelevation above the bed of particles but beneath the reactor corewhereby vapor can be vented from the porous bed.
 2. The nuclear reactorof claim 1 wherein the safety apparatus includes a plurality of tie rodssecured to the elongated tubular elements adjacent their upper endsoperable for supporting the elements in a lateral array.
 3. The nuclearreactor of claim 1 wherein the safety apparatus includes means forprecluding falling debris from entering the elongated tubular elementswhile allowing passage of coolant vapor.
 4. The nuclear reactor of claim3 wherein the precluding means includes means for covering eachelongated tubular element at the upper end thereof to preclude debrisfrom falling therein.
 5. The nuclear reactor of claim 4 wherein saidcovering means is in the form of an elbow secured to the elongatedtubular element over the upper end thereof operable to define asidewardly disposed opening from the interior passageway of the tubularelement.
 6. The nuclear reactor of claim 4 wherein said covering meansis in the form of a cap secured to the elongated tubular elements has aplurality of holes therein near the cap that define sidewardly orientedopenings from the interior passageway of the tubular element.
 7. Thenuclear reactor of claim 4 wherein said covering means is in the form ofa plurality of a small particles randomly stacked on one another withinthe interior passageway of the elongated tubular element.
 8. The nuclearreactor of claim 7 wherein said particles define a void fraction of theorder of between 0.25 and 0.75.
 9. The nuclear reactor of claim 8wherein said particles are of about the same size, shape and material asthe particles forming the bed in the confinement vessel.